Cassettes typically hold a variety of instruments for cleaning of gross residues by immersion in a bath through which ultrasonic energy is passed in a liquid vehicle. The instruments are subsequently sterilized in a separate procedure frequently involving autoclave steam sterilization. Transmission of the vibrations into contact with the instruments is essential to effective cleaning. Improvements in instrument cleaning cassettes to lower cost, improve handling and durability are desirable but such improvements should not come at the cost of lessened cleaning effectiveness. Nor should excessive condensate after steam sterilization be endured. Metal cassettes are heavy and awkward to handle and typically have sharp edges which can readily tear enclosure pouches, generally formed of plastic and cellulosic films, in which the cassettes and their contents are sterilized and stored for use. Failure of the cassette to dissipate sterilization steam without excessive condensation can cause moisture damage to the pouch and allow breach of sterile conditions in the pouch through wicking where excessive water is present.